Personal communication systems (PCS) operate using various protocols and standards of operation. These communication systems are often implemented in more than one country each having a separate governing body overseeing communication standards and regulations. In order for a communication system to be implemented into a country, the system must adhere to the standards specific to that country. Cordless telephone second generation (CT2) communication systems operate in accordance with a document entitled "MPT1375 Common Air Interface Specification" (CAI), Version prI-ETS 300 131, dated 31st Jan. 1994, which is published by the European Telecommunications Standards Institute and is hereby incorporated by reference. The CAI establishes a time-division multiplexed protocol having alternating one millisecond receive and transmit frames separated by guard time segments and is a standard adopted by most of the European community.
In a typical CT2 system, the communication protocol standard includes four main burst structures, called multiplex 3 (MUX3), multiplex 2 (MUX2), and multiplex 1 (MUX1) which is further subdivided into either multiplex 1.4 or 1.2 (MUX1.4 or MUX1.2). MUX3 is utilized mainly for communication link initiation (link establishment and re-establishment) between a portable radio and a base station. The MUX2 protocol employs a synchronization and data information exchange multiplex and is used primarily for communication link establishment and for link initiation from the base station. The MUX1 protocol is used primarily for voice/data communications, signaling information, and control messages between the portable to the base.
The current CAI protocol provides the following method for a portable radio to initiate a communication link to a base station. The portable selects a free channel and begins to transmit a non-synchronous link request using the MUX3 data format which has a transmit-to-receive ratio of 10:4. The base scans all frequencies in search of a link request. Because a base cannot synchronize to a portable, the structure of MUX3 is such that a base may examine slices of the transmitted data for the link request. Once the link request is recognized, the base begins to transmit a link grant to the portable in the MUX2 format. The portable synchronizes to the base's MUX2 transmission, recognizes the link grant, and terminates its MUX3 transmissions. The portable then begins to transmit in the MUX2 format using the base's synchronization. At this point a link is established in the MUX2 format, as well as the subsequent MUX1.2 format and MUX1.4 format, which has a transmit-to-receive ratio of 1:1.
When a base initiates a link to a portable, it foregoes the MUX3 format and uses the, MUX2 format immediately. The portable scans all frequencies in search of an incoming call poll. Because the portable synchronizes to base transmissions, the MUX2 format can be read directly. Once the portable recognizes the poll, it responds with a link request in MUX2 which is subsequently answered by the base with a link grant. At this point a link is established in the MUX2 format, as well as the subsequent MUX1.2 format and MUX1.4 format, which has a transmit-to-receive ratio of 1:1.
When a portable initiates a link, it addresses a specific base or service (a specific grouping of bases) using a unique identification code which only the targeted base or service will recognize. Likewise, when a base initiates a link it addresses a specific portable using a unique identification code which only the targeted portable will recognize. Though it is possible for a base to target more than one portable by interleaving different codes, it must target each portable individually with the portable's unique code. This can become a problem, especially in a public base environment where thousands of portable radios, each having their own unique identification, code may be used.
In the United States the MUX3 format violates the transmit-to-receive ratio rule established by the Federal Communications Commission (FCC)which requires a frame length for unlicensed PCS equipment of 10 ms/N (where N is an integer.gtoreq.1). Accordingly, the use of MUX3 for CT2 portable initiated calls must be avoided. Furthermore, non-synchronous transmissions, such as MUX3, create interference which can degrade performance and/or traffic capacity. A system using synchronous transmissions would reduce interference and improve system capacity. Hence, there is a need for an apparatus and method that provides and supports a synchronous link initiation from a portable to a base.